Hydraulic fracturing is a stimulation treatment routinely performed on oil, gas and other wells to increase fluid production from reservoirs. Specially engineered fluids are pumped at high pressure and rate into the reservoir interval to be treated, causing a fracture to open. Proppant, such as grains of sand of a particular size, is slurried with the treatment fluid (usually therefore called a carrier fluid) to keep the fracture open when the treatment is complete. Hydraulic fracturing creates high-conductivity communication with a large area of a formation and bypasses any damage that may exist in the near-wellbore area. Combined fracturing and gravel packing operations are used extensively to stimulate poorly consolidated sand formations. The goal of combined fracturing and gravel packing treatments is to create short, wide, highly conductive fractures and then to pack the area between the screen and the perforated casing or the inside wellbore surface (the sandface) by continuing to inject slurry after the fracture has filled with proppant and packed off. Short, wide fractures might also be desirable in wellbores and formations in which sand control is not an issue and so gravel packing is not performed. An example would be in relatively high permeability formations in which fracture conductivity is particularly important. The most common method of creating short, wide fractures is to initiate a tip screenout during the pumping operation. In a tip screenout, the solids concentration at the tip of the fracture becomes so high due to fluid leak-off into the formation that the slurry is no longer mobile. The concentrated proppant slurry plugs the fracture, preventing additional growth of the fracture length. Additional pumping of the proppant/fluid slurry into the formation after the screenout occurs causes the fracture to balloon. The fracture grows in width rather than length, and large concentrations of proppant per surface area are placed in the fracture. The design of these treatments relies heavily on knowing the correct mechanical, permeability, reservoir pressure and fluid saturations of the formation being treated and the properties of the fluids and slurries being pumped.
The only ways currently known to cause, or increase the probability of, a tip screenout are by manipulation of parameters such as these. U.S. Pat. No. 5,054,554 describes several ways of inducing tip screenouts. In one, when the fracture has reached the desired length and a tip screenout is desired, the proppant concentration and slurry pumping rate are both lowered and the proppant concentration is then raised. In another, the proppant concentration is progressively increased until tip screenout occurs. U.S. Pat. No. 5,325,921 describes a method of inducing a tip screenout in which a very effective fluid loss additive is used in the pad, but a less effective fluid loss additive is used in the proppant stages such that leak-off becomes sufficiently high as to result in a screenout.
Prior to most hydraulic fracturing or fracturing/gravel packing treatments a small fracturing treatment (sometimes called a “data frac” or “mini-frac”) is performed in order to measure the needed parameters and to determine the formation's response to a hydraulic fracturing treatment. Treatment designs are often modified on the fly to incorporate this new information. Treatment design and modification is typically done with the aid of a computer model, many of which are available in the industry.
Unfortunately, in spite of data-fracturing information, the pressure transients collected by downhole pressure gauges during treatments indicate that tip-screenouts do not occur in many, perhaps the majority, of the fracturing or combined fracturing/gravel packing treatments in which they are desired and intended. The fluid at the tip of the fracture remains mobile, the fracture tip continues to grow throughout the treatment and the desired fracture dimensions and proppant concentration in the fracture is not reached. Therefore, the desired fracture conductivity is not obtained. Often, tip screenouts have to be coaxed by, for example, dropping pump rates at the time a tip screenout is desired.
There is a need for a more reliable way to ensure that intended tip screenouts will occur where they are wanted in stimulation treatments and to allow for more flexibility in design of stimulation treatments in which tip screenouts are desired.